A neck of this type, which generally has a thickness of the order of 1 mm. and consists of a material which melts upon the application of heat such as polyethylene, is generally sealed in an air-tight manner by the action of heat, this neck being compressed in the form of two walls placed side-by-side between two solid jaws heated to an adequate temperature to ensure melting and welding of the material in the area to be sealed.
Such a method, which comprises heating the contact area of the two walls placed side-by-side, i.e. in reality the only area for which there is any purpose in causing melting, with a view to its welding, through these walls, comprises several drawbacks.
One of these drawbacks resides in the very difficult control of the flow of the molten material as the jaws penetrate the latter, a drawback which is further increased by frequent irregularities in thickness of the neck over its periphery and by the difference in thickness frequently found from one neck to another.
Moreover, it is difficult to determine an exact optimum welding time in view of these differences in thickness from one neck to another or on the periphery of the same neck.